I took my son on his first trip when he was 2 months old and I know those worries you are having.
There is a pull down changing table in the airplane toilet and you can easily change your baby's diapers there.
It's always a challenge for baby to sleep on planes and mine did not had a wink for a 4 hour flight to HongKong and only 6 hours of sleep when we fly to Toronto when the entire journey was 20 hours. I would normally play with my son till he's tired and he will fall asleep eventually. I will avoid any sorts of drug to make him sleep unless he gets really cranky and make huge disturbances to other passengers on long haul flights.
I would normally put some water in the milk bottle and let my son drink from it and that eases the discomfort from the pressure. You might want to try.
You can visit my blog for some travel tips if you like. www.curlyray.com